Posted: Sat, Jan 5, 2013, 9:45 AM ET (1445 GMT)
An extrapolation based on data from NASA's Kepler spacecraft has led astronomers to conclude that there are at least 100 billion, and perhaps 200 billion, planets orbiting stars in the Milky Way galaxy. The team of Caltech astronomers based their analysis on the discovery of five planets orbiting a star designated Kepler-32, an M-class star that is among those being observed by NASA's Kepler planet-hunting spacecraft. Using the probability that the Kepler-32 system would be oriented in a way that allowed its detection, astronomers concluded that there is an average of one planet per star in the Milky Way, or at least 100 billion planets; taking into account planets orbiting stars at greater distances than those spotted around Kepler-32 could double that total, scientists said. The new estimate closely matches previous estimates, but is the first to take into account planets orbiting M-class stars, which account for approximately three-quarters of the stars in the galaxy.